1. Field of the Invention
This invention relates to an electronic ignition control system for use in internal combustion engine applications. More particularly, it relates to such an ignition control system having an improved feedback loop for dwell control.
2. Description of the Prior Art
Conventional or Kettering automobile ignition systems make use of mechanical breaker points which are periodically opened by cams to interrupt the passage of electrical current through the ignition coil, thus inducing high voltage signals in the secondary winding of the coil, which provide the spark at the plugs for ignition. A primary problem with such an electro-mechanical system is that the breaker points have a limited life expectancy; consequently, there is a need for periodic tune-ups to keep the engine running smoothly.
As semiconductor device and integrated circuit technology has developed, it was soon perceived that integrated circuits could be used as electronic substitutes for the cam operated breaker points and their associated condenser. The initial thrust of electronic ignition control systems has been to simply duplicate the performance characteristics of the conventional system. An example of such an electronic ignition system is disclosed in Arguello, U.S. Pat. No. 4,057,740. In that system, a comparator circuit compares an input signal from a sensing means in a distributor against a constant reference voltage and produces output signals when the input signal exceeds the constant reference voltage.
It is further recognized in the electronic ignition control system art that electronic ignition systems need not be limited by certain design compromises in the signals produced using mechanical breaker points because mechanical wear is not a problem in an electronic system. For example, Adamian et al. in U.S. Pat. No. 3,882,840 disclose an electronic ignition system in which a higher primary energy may be employed with the ignition coil then has been found practical with mechanical breaker points. This consideration in particular is of great significance for present day automobile engines, which operate with leaner combustion mixtures than previously employed, both for fuel economy and anti-pollution reasons.
One aspect of the system of Adamian et al. is a feedback loop from the output of the system there disclosed to a dwell control circuit. This feedback loop allows variation in dwell angle or dwell time for different engine operating conditions. However, such a long feedback loop makes it difficult to achieve tight control over dwell, particularly as the control requirements become more stringent with further demands for increased fuel economy and pollution control in internal combustion engines.